Final answer:
Water flows over gills in the opposite direction of blood flow in a process known as countercurrent exchange. This process allows for efficient gas exchange between water and blood in fish gills.
Step-by-step explanation:
The process by which water flows over gills in the opposite direction of blood flow is known as countercurrent exchange. This process allows for efficient gas exchange between the water and blood in fish gills.
As water flows over the gills, oxygen from the water diffuses into the blood with a low concentration of oxygen molecules. At the same time, carbon dioxide from the blood diffuses into the water. This exchange occurs due to the concentration gradient between the water and blood.
Countercurrent exchange enhances the efficiency of gas exchange by maintaining a concentration gradient along the entire length of the gill filaments, ensuring that oxygen diffuses from the water into the blood throughout the entire gill surface.